New telecommunications services recently introduced and presently being planned require that high-speed terminating equipment be interconnected to telecommunications service points using high capacity digital facilities. Accordingly, there is currently special interest in high rate digital subscriber lines (HDSL) to carry 1.544 Mbit/sec bidirectional data over local copper telephone loops. As originally proposed, HDSL utilized two twisted wire pairs for transmission of the 1.544 Mbit/sec over the required distance range with an acceptable error rate. The basic communication environment of HDSL then is one wherein there are two channels available for transmission and each channel has additive noise with different power. Moreover, the transmitter has no knowledge of the power of the noise on each channel. Also, the noise powers are constant or vary slowly so that the receiver may estimate the noise statistics.
The initial implementation of HDSL utilized so-called "uncoordinated" transmission to meet the acceptable error rate criterion, but oftentimes only marginally. With uncoordinated transmission, the total message represented by a data stream is divided and each channel independently carries a part of the stream. Uncoordinated transmission has good performance when each channel has about the same noise power, but if just one channel is very noisy then the message is lost. Another technique deployed to overcome limitations of uncoordinated transmission is called "symbol splitting" diversity. With symbol splitting diversity, a copy of the entire message is transmitted on each channel so that the message may still be fully recovered if one channel is very noisy. However, symbol splitting has relatively poor throughput performance if each channel has nearly the same noise power.
Recently, in a paper entitled "Coordinated Transmission for Two-Pair Digital Subscriber Lines, " IEEE Journal on Selected Areas in Communications, Special Issue on High-Speed Digital Subscriber Lines, Vol. 9, pp. 920-930, August, 1991, J. W. Lechleider proposed an improvement to the performance of HDSL by coordinating the transmission on the two lines. In order to achieve the required improvement in the signal-to-noise ratio as well as other enhancements and benefits, the coordinated transmission technique relates loop signal amplitudes transmitted on the two lines to each other and processes the received signals according to the prescribed relation between the amplitudes. The system for coordinated transmission includes both a central office transceiver and a remote transceiver terminating the corresponding ends of the pair of lines. Each transceiver includes a near-end transmitter and a receiver for a far-end transmitter. The near-end transmitter distributes first and second data signals originated by first and second data sources, respectively, over the pair of transmission lines. The lines are coupled to this transmitter by first and second hybrid arrangements which each have transmit and receive ports. The receive ports of the hybrid arrangements provide first and second received line signals, respectively, each having echo and NEXT (Near-End Crosstalk) interfering components. Each transmitter in the coordinated transmission system is adaptive and requires knowledge of the noise on the transmission lines. The statistics are formed at the far-end receiver and are fed back to the near-end transmitter so that the amplitudes and power of the transmitted data signals on the first line are typically different than the amplitudes and power on the second line.
Another method of implementing coordinated transmission using an adaptive transmitter was also considered by Lechleider in "Averaging of the Signal to Noise Ratios of the Constituent Pairs of a Two-pair HDSL," ECSA Contribution, T1E1.4.89, Dec. 11, 1989. In this Contribution, Lechleider's adaptive matrix transmitter essentially whitens the noise, and leads to the optimal "water-filling" power allocation. D. D. Falconer, in his article "Simple Reception Techniques for Coordinated Two-Pair Digital Transmission," 1991 Globecom Conference, showed that significant performance gains can be achieved simply by allowing the average transmitted signal power on each channel to vary, while keeping the sum of the transmitted power on all the channels constant.
As pointed out above, the techniques taught or suggested by the prior art relating to coordinated transmission systems have only achieved the aforementioned improvements by utilizing adaptive transmitters. Adaptive transmitters have the disadvantage of requiring that knowledge of the channel response be supplied to the transmitter, and moreover, such transmitters send different average powers on different channels.